Add declarative IsHitTestVisible(bool) common element modifier

[nsreehari]

Summary

Adds a declarative IsHitTestVisible(bool) modifier to the common element surface, filling the last gap among the sibling boolean/visual state modifiers (IsEnabled, IsTabStop, IsVisible, Opacity). Until now, opting an element out of pointer hit-testing required the imperative .Set(c => c.IsHitTestVisible = false) escape hatch.

Fixes #809.

Motivation

Reactor.Community.ReactorFlow's MiniMap renders pointer-transparent overlay layers (viewport rectangle, node thumbnails) that must not intercept pointer input destined for the interactive canvas underneath. Its last remaining .Set(...) escape hatch is a NonInteractive helper whose entire body is element.Set(c => c.IsHitTestVisible = false). A declarative modifier lets that helper be dropped and the property composed like any other element state.

What changed

Plumbing mirrors IsTabStop exactly:

• src/Reactor/Core/Element.cs — added bool? IsHitTestVisible to ElementModifiers, folded into ModifiersEqual and Merge.
• src/Reactor/Elements/ElementExtensions.cs — added public static T IsHitTestVisible<T>(this T el, bool isHitTestVisible = true) where T : Element next to IsTabStop, with matching XML-doc style.
• src/Reactor/Core/Reconciler.cs — applies fe.IsHitTestVisible = value only when set and changed, next to where IsTabStop is applied.
• tests/Reactor.Tests/ElementExtensionsCoverageTests.cs — added IsHitTestVisible_Sets_Modifier, mirroring IsEnabled_Sets_Modifier.
• skills/reactor.api.txt + plugins/reactor/skills/reactor-dsl/references/reactor.api.txt — regenerated API index (UPDATE_API_INDEX=1).

Additive and back-compat: new optional modifier defaulting to true (the WinUI default); no existing signature or behavior changes. Recycle-safety is already covered — ElementPool clears UIElement.IsHitTestVisibleProperty on release (issue #162), so a recycled control cannot strand a stale false.

Validation

Built and tested on win-x64:

UPDATE_API_INDEX=1 dotnet test tests/Reactor.Tests \
  --filter "FullyQualifiedName~Tooling.ApiIndexGeneratorTests.Index_IsUpToDate" -r win-x64
# → regenerated both reactor.api.txt copies; Passed 1

dotnet test tests/Reactor.Tests \
  --filter "FullyQualifiedName~ElementExtensionsCoverageTests|FullyQualifiedName~Tooling.ApiIndexGeneratorTests" \
  -r win-x64 --no-build
# → Passed 63, Failed 0 (incl. IsHitTestVisible_Sets_Modifier and Index_IsUpToDate)

Build succeeded with 0 errors.